Using non-spherical particles as abrasive grains in a polishing composition is effective as a way of enhancing the rate of polishing with the polishing composition. Japanese Laid-Open Patent Publication No. 2007-153732 (Patent document 1) discloses the use of colloidal silica particles having a number of small protrusions on each surface thereof in mirror polishing of a silicon wafer. Japanese Laid-Open Patent Publication No. 2009-149493 (Patent document 2) discloses a polishing composition in which silica sol containing silica particles having a minor axis/major axis ratio in the range of 0.01 to 0.8 and having wart-like protrusions on each surface thereof is used as abrasive grains.